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From Linear Molecular Chains to Extended Polycyclic Networks: Polymerization of Dicyanoacetylene Huiyang Gou, LI Zhu, Haw-Tyng Huang, Arani Biswas, Derek W. Keefer, Brian L Chaloux, Clemens Prescher, Liuxiang Yang, Duck Young Kim, Matthew D Ward, Jordan Lerach, Shengnan Wang, Artem R Oganov, Albert Epshteyn, John V Badding, and Timothy A. Strobel Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b01446 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on June 26, 2017
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Chemistry of Materials
From Linear Molecular Chains to Extended Polycyclic Networks: Polymerization of Dicyanoacetylene Huiyang Gou,1,2,*,† Li Zhu1,*, Haw-Tyng Huang,3 Arani Biswas,3 Derek W. Keefer,3 Brian L. Chaloux,4 Clemens Prescher,5 Liuxiang Yang, 1,2 Duck Young Kim,1,2 Matthew D. Ward,1 Jordan Lerach,3 Shengnan Wang, 6 Artem R. Oganov, 7,6 Albert Epshteyn,8 John V. Badding,3 Timothy A. Strobel1,‡ 1
Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA Center for High Pressure Science and Technology Advanced Research, Beijing 100094, China. 3 Department of Chemistry, Department of Physics, Department of Materials Science and Engineering, and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA. 4 NRC Postdoctoral Associate, Naval Research Laboratory, 4555 Overlook Ave., SW, Washington, DC 20375, USA 5 Center for Advanced Radiation Sources, University of Chicago, Argonne, IL 60437, USA 6 Department of Geosciences, Center for Materials by Design, and Institute for Advanced Computational Science, State University of New York, Stony Brook, NY 11794-2100 7 Skolkovo Institute of Science and Technology, Skolkovo Innovation Center, 3 Nobel St., Moscow 143026, Russia. 8 Naval Research Laboratory, Washington, DC20375, USA 2
*
Contributed equally to this work
[email protected] ‡
[email protected] †
ABSTRACT Dicyanoacetylene (C4N2) is an unusual energetic molecule with alternating triple and single bonds (think miniature, nitrogen-capped carbyne), which represents an interesting starting point for the transformation into extended carbon-nitrogen solids. While pressure-induced polymerization has been documented for a wide variety of related molecular solids, precise mechanistic details of reaction pathways are often poorly understood and the characterization of recovered products is typically incomplete. Here, we study the high-pressure behavior of C4N2 and demonstrate polymerization into a disordered carbon-nitrogen network that is recoverable to ambient conditions. The reaction proceeds via activation of linear molecules into buckled molecular chains, which spontaneously assemble into a polycyclic network that lacks long-range order. The recovered product was characterized using a variety of optical spectroscopies, X-ray methods and theoretical simulations, and is described as a predominately sp2 network comprising “pyrrolic” and “pyridinic” rings with an overall tendency towards a two-dimensional structure. This understanding offers valuable mechanistic insights into design guidelines for nextgeneration carbon nitride materials with unique structures and compositions.
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INTRODUCTION Carbon nitride materials have attracted continuous attention in research over past decades as indicated by extensive efforts from both theory1-2 and experiments3-19. Sustained interest in nitrogen-bearing, carbon-rich materials stems from their multiple functionalities and diverse applications.1-19 For example, cubic carbon nitride (c-C3N4) is predicted to have comparable hardness to diamond,1-2 whereas graphitic carbon nitride (g-C3N4) has been demonstrated to act as a photocatalyst for various redox reactions.4-19 Furthermore, nitrogen-doped carbon materials often exhibit excellent properties as compared with their pure carbon counterparts, e.g. conductivity, basicity, oxidation stability, and catalytic activity, although these properties are highly dependent on the amount of nitrogen incorporated within the structure.4-19 When longrange order within carbon nitride materials is lacking, disordered structures containing sp2 and sp3 bonding may still exhibit outstanding mechanical properties,20-26 which is fundamentally important for the realization of three-dimensional, crystalline carbon nitride solids. Previous attempts to produce novel CN solids have largely revolved around varying deposition methods to manipulate particle size and texture, together with control of nitrogen content.10-17 Nevertheless, adventitious hydrogen incorporation within these materials limits the development of structure-property relationships for precise stoichiometries and diverse CN building blocks.2732 High-energy molecules can provide an alternative path towards the formation of unique structures, and high-pressure chemistry may be a viable approach to obtain novel extended CN networks with predetermined architectures.33 Studies of the polymerization reactions of cyanogen,34 cyanoacetylene,35 tetracyanoethylene,36 phosphorous tricyanide,37 and acetonitrile38 under pressure have helped to clarify the influences of starting materials and reaction conditions for the synthesis of new CN materials. Dicyanoacetylene (C4N2) is a linear molecule with alternating triple and single bonds (N≡CC≡C-C≡N).39 The standard enthalpy of formation of C4N2 is 500 kJ/mol, 40 leading to a flame temperature of over 5000 K.41 Thus, C4N2 represents an interesting, high-energy starting point to access novel extended CN networks. From the low-pressure side, people have examined C4N2 and found reactions with benzene to produce aromatic hydrocarbons via the Diels-Alder reaction, 42 in addition to explosive decomposition into dinitrogen and graphite. Chien and Carlini synthesized polydicyanoacetylene via anionic polymerization of C4N2 with n-butyllithium in THF, producing a linear polymer that cyclizes into a ladder polymer upon heating at 400 °C. 42 Interestingly, the physiochemical behavior of C4N2 is also of astrophysical importance due to its presence in Titan’s north polar stratosphere,44-47 as detected by infrared spectroscopy. However, the interpretation of Raman and IR spectra is challenging, especially with regards to conflicting assignments for C≡N and C≡C stretching frequencies.48-54 Inconsistencies between spectroscopic assignments for C4N2 may arise from difficulties associated with sample preparation and handling due to its inherent instability and high reactivity; although it is of great importance for the understanding of intrinsic properties and conclusions regarding its detection. To our knowledge, C4N2 has not yet been examined under high-pressure conditions, which is of great interest from the perspective of novel carbon nitride materials and fundamental chemical 2
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Chemistry of Materials
transformations. The large fraction of unsaturated bonds suggests potential for pressure-induced ring-forming reactions, as has been documented for a large number of other nitrile- and alkynebased systems. 34-38 In addition, C4N2 represents an interesting C:N stoichiometry to access extended networks with nitrogen content intermediate between typical deposition-based syntheses (nitrogen doping within a carbon framework) and precursor-based systems like graphitic C3N4. Moreover, high-pressure behavior can help to understand the nature of chemical bonding, reaction mechanisms, atomic structure and local environments of reaction products, which will advance the general understanding of carbon nitride materials. Here, we study the high-pressure behavior of linear dicyanoacetylene (C4N2) up to 10 GPa in diamond anvil cells. We first resolve the standing controversy regarding assignments of Raman and IR spectra, aided by first-principles phonon spectrum calculations. We next uncover a pressure-induced reaction process whereby discrete linear molecules polymerize into a disordered extended network without significant change to the bulk composition. This novel, amorphous material is fully recoverable to ambient conditions and its local structure, composition and chemical bonding was established using a variety of optical and X-ray scattering methods. The reaction mechanism was rationalized using molecular dynamics simulations. The present results provide a framework to further the understanding of the local mechanism of polymerization and offer valuable mechanistic insights into design guideline for the next-generation carbon nitride materials. METHODS Synthesis C4N2 was prepared according to literature procedures. 55 All reactions were performed under an inert atmosphere of either nitrogen or argon. Diethyl ether was dried and deoxygenated over sodium-benzophenone ketyl prior to use. Sulfolane was purified by reaction with KMnO4 to remove alkene impurities, dried over KOH, and vacuum distilled prior to use. Acetylene dicarboxamide (1). 100 mL of NH3 (4.8 mol) were condensed into a Schlenk flask and chilled to -78 °C. Separately, 13.41 g (94.36 mmol) of dimethyl acetylenedicarboxylate (Sigma-Aldrich, 99%) were dissolved in 100 mL anhydrous diethyl ether (Et2O) and transferred dropwise to the chilled flask containing NH3; the reaction mixture quickly developed a red coloration. The reaction mixture was warmed to -45 °C and stirred for 16 h at this temperature. Residual NH3 was boiled off at ambient temperature and the remainder was evaporated to dryness under vacuum, yielding a crude, orange product that was subsequently washed with cold ethanol. The resulting beige powder was recrystallized from 150 mL boiling ethanol at -20 °C, yielding 8.27 g (78% yield) of white, crystalline solid after filtering and drying the powder under vacuum. Dicyanoacetylene (2). 125 mL anhydrous sulfolane was added to a Schlenk flask equipped with a solids addition funnel and a secondary (detachable) vacuum trap. Using a mortar and pestle, 3
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1.960 g (17.49 mmol) of acetylene dicarboxamide (1) was ground with 15.456 g (54.06 mmol) P2O5 until homogeneous. Solids were added to the addition funnel, the apparatus was evacuated, the trap was chilled to -196 °C, and the sulfolane was heated to 110 °C with vigorous stirring. Solids were added slowly to the reaction mixture, which changed from colorless to yellow to red over time. After bubbling had ceased (~3 h), the trap containing the crude product was removed, transferred to an argon-filled glovebox, and liquid was pipetted into a small H-tube after melting. The liquid was frozen with LN2, a static vacuum of 50 mTorr was applied on the assembly. The dewar was transferred to the empty arm of the H-tube and the product was sublimed by warming the arm. The volatile, pale beige (almost colorless) liquid product was weighed within the glovebox at 0.794 g (60% yield). The product was stored at -35 °C under argon to prevent decomposition. Poly-C4N2. Liquid dicyanoacetylene (2) (~1 µl) was loaded into a ~150-210 µm diameter hole in a Re gasket using clean microsyringe. Before sample loading, Re gaskets were pre-indented to a thickness of 60-80 µm and mounted on 300-500 µm anvil culets of symmetric diamond anvil cells (DAC). Pressure was determined by measurement of fluorescence from a ruby standard placed inside the gasket hole.56 To avoid any possible contamination of the highly reactive C4N2, a pressure medium was not used in the runs, but the molecular crystals are very soft and the influence of deviatoric stresses is expected to be small. All sample loadings were conducted in an inert Ar gas atmosphere glove box with oxygen and moisture concentrations of less than 1 ppm. All samples were sealed to a starting pressure of ~0.1 GPa within the inert Ar atmosphere before removing from the glove box. Raman spectroscopy A system based around a Princeton Instruments spectrograph SP2750 (Trenton, NJ) with a 750 mm focal length was used for the Raman spectra collections. A 532 nm diode laser was used as an excitation source and was focused through a 20× long working distance objective lens. The laser power was optimized to be ~1 mW and a short exposure time of ≤60 s was used to avoid local heating of the sample. Raman light was collected in the backscatter geometry through a 50 µm confocal pinhole and two narrow-band notch filters (Ondax) were used to allow collection to within ~10 cm−1 from the laser line. Raman light was collected though a 50 µm slit and dispersed off of an 1800 or 300 gr/mm grating onto a liquid nitrogen cooled charge coupled device (CCD) detector providing a spectral resolution of about 2 cm−1. The spectrometer was calibrated using the emission lines of Ne with an accuracy
